The molecular sieves of interest to this invention are crystalline metal aluminosilicates having a general formula EQU M.sub.2/n O:Al.sub.2 O.sub.3 :xSiO.sub.2 :yH.sub.2 O
where values for x and y vary according to the crystalline structure of the particular zeolite. These materials consist basically of a rigid three-dimensional lattice of silica and alumina tetrahedra crosslinked through oxygen atoms. To the extent that the alumina and silica anions are not charge balanced, cations, represented by M in the above formula, occupy charged sites on the internal surface of the lattice. Typical cations include ammonium, protons, alkali metals, alkaline earth metals, transition metals and rare earth metals in oxidation states of one, two or three. Water molecules often occupy spaces between the tetrahedra unless the crystal has been dehydrated.
Many procedures for synthesizing zeolites produce materials that are very fine, often less than a few microns in size. The small particle size powders are difficult to use in many industrial processes. Furthermore the small sizes can also create dust hazards for people who handle the materials. Larger zeolite particles or so-called massive bodies, ranging in size from several microns to one fourth inch in their largest dimension, are clearly preferred in many applications. However, they must retain the ion-exchange properties, adsorption capacity and selectivity, thermal stability and catalytic activity of the finely divided crystalline zeolite. In addition, the particles should exhibit high attrition resistance and crush strength.
Larger bodies having sizes in excess of five microns can be prepared by agglomerating small crystals. A typical process requires a suitable binder, such as a clay or inorganic or organic adhesives, and processing conditions that assure reproducibility of the properties of the agglomerates. Because such processing conditions are often complex and difficult to control and because the binder material reduces the adsorptive and catalytic properties of the zeolite by dilution and other means, this approach is not optimal.
An alternative procedure for making particles whose largest dimension is as high as one fourth inch begins with the preparation of a precursor, or preformed body, which contains certain reactive or unreactive kaolin-type clays and which can be converted by chemical means to a zeolite body that retains the shape of the preformed body. However, the resulting massive bodies prepared by this method frequently, and often unpredictably, exhibit poor crush strength and/or adsorption properties. In addition, the wet strength of the preformed bodies made by prior art methods is generally quite low; the preferred bodies that are converted to zeolite often disintegrate during aging and digestion, especially if any agitation is used. As a result a substantial quantity of fine zeolite powder is usually formed. In addition, the rates of zeolite formation are generally slow, requiring at least one day in actual practice; most of the examples in the prior art methods require three days.
It is, therefore, a primary object of this invention to provide an improved process for producing molecular sieves in massive bodies whose zeolite content and crush strength are uniformly high and predictable.
It is additionally an object to provide a method of controlling the porosity of the preformed body in order to control and enhance the rate of zeolite formation.
It is a further object to provide preformed bodies whose wet strength is high.